ADAU1452 pass transistor power dessipation

Hello Archana,

I'm not sure where you got the number 40 mA, but that's not the expected maximum DVDD current for the ADAU1452. The datasheet will be available on the website in the next week or so, but for now, here are a few excerpts from the specification table.

At a temperature of 25 C...

Digital Current (DVDD)

Min

Typ

Max

Operation State

Maximum Program

350

415

mA

Typical Program

100

mA

Test program includes 16-channel I/O, 10-band EQ per channel, all ASRCs active

Minimal Program

85

mA

Test program includes 2-channel I/O, 10-band EQ per channel

Idle State

20

95

mA

Power applied, DSP not enabled

Reset State

20

95

mA

Power applied, held low

At an ambient temperature of up to 105 C...

Digital Current (DVDD)

Min

Typ

Max

Operation State

Maximum Program

500

690

mA

Typical Program

200

mA

Test program includes 16-channel I/O, 10-band EQ per channel, all ASRCs active

Minimal Program

160

mA

Test program includes 2-channel I/O, 10-band EQ per channel

Idle State

315

635

mA

Reset State

315

635

mA

So, as you can see, the absolute worst case current can be up to 690 mA.

In the section of the datasheet describing the regulator, we recommend the following transistor specs:

For proper operation, the linear regulator requires several external components. A PNP bipolar junction transistor acts as an external pass device to bring the higher IOVDD voltage down to the lower DVDD voltage, thus externally dissipating the power of the IC package. Ensure that the current gain of the transistor (β) is 200 or greater and the transistor is able to dissipate at least 1 W in the worst case. Place a 1 kΩ resistor between the transistor emitter and base to help stabilize the regulator for varying loads. This resistor placement also guarantees that current is always flowing into the VDRIVE pin, even for minimal regulator loads.

Hi BrettG,

Thank you. My bad, assumed wrong value. If I take the 690mA as worst current, the power dissipation calculation will be (3.3-1.2)*690mA = 1.5W (approx)

Is this the correct way to determine worst case PD in the external transistor? Do you see any other factors which can affect PD significantly.

Thank you.

Regards,

Archana.

Hi Archana,

It's not explained very well in those tables, but the 690 mA number is very much a result of "stress test" conditions and quite a bit of guard-banding. In a real world system, you shouldn't see DVDD current exceed 500 mA. However, it's always better to be safe and build in some extra safety margin. So, I agree with your calculation.

I believe our selected transistor for the evaluation board has a very significant safety margin. Here are its specs:

Above transistor specs will definitely help me to check the transistor selected for our design.

Thank you BrettG. This helped me a lot.

Thanks,

Archana.